Abstract The constancy of the speed of light is traditionally treated as a postulate of special relativity. This paper provides a constitutive derivation of this postulate by decomposing spacetime into a hierarchical structure: a substrate field Fₛub encoding the electromagnetic constitutive parameters (epsilon₀, mu₀), and an excitation field Fₑxc describing propagating signals. We demonstrate that the source-independence of light speed follows from the near-vanishing variational coupling delta Lₑxc / delta Fₛub approx 0 over observational scales. The closure condition (cepsilon₀) (cmu₀) =1 is derived as an emergent infrared attractor, with O (Lambda^-1) corrections at high energies. The Michelson-Morley null result is reinterpreted as a symmetry-protected cancellation between constitutive anisotropy and coordinate rescaling. This framework bridges the gap between formal relativity and constitutive physics, providing a physical mechanism for the light-speed postulate without modifying its empirical success. Keywords: light-speed postulate; constitutive physics; hierarchical field decomposition; spacetime substrate; Michelson-Morley experiment; CSSD Series 摘要 光速不变性在传统上被视为狭义相对论的一个基本假设。本文通过将时空分解为层级结构, 为这一假设提供了本构推导: 即编码电磁本构参数 (epsilon₀, mu₀) 的基底场 Fₛub, 以及描述传播信号的激发场 Fₑxc。我们证明, 光速对于波源的独立性源于在观测尺度上趋于零的变分耦合 delta Lₑxc / delta Fₛub 约等于 0。闭合条件 (cepsilon₀) (cmu₀) =1 被导出为一个涌现的红外吸引子, 并在高能条件下带有 O (Lambda^-1) 的修正。迈克尔逊-莫雷实验的零结果被重新诠释为本构各向异性与坐标重标度之间受对称性保护的抵消。该框架弥合了形式相对论与本构物理之间的鸿沟, 在不改变其经验成功的前提下, 为光速不变假设提供了物理机制解释。 关键词: 光速不变假设;本构物理;层级场分解;时空基底;迈克尔逊-莫雷实验;CSSD 系列
Hugang Cui (Thu,) studied this question.